1. Field of the Invention
The present invention is related to an optical amplifier employing an optical fiber that is doped with a rare-earth metal, such as erbium (Er), and in particular to an optical amplifier for collectively amplifying optical signals having a plurality of multiplexed wavelengths.
2. Related Arts
An optical amplifier that contains an optical fiber that is doped with erbium (Er) has been adopted for optical communication. Furthermore, to cope with increases in the capacities of optical communication systems, progress has been made in the development of a technique for collectively amplifying optical signals having multiple wavelengths wherein a plurality of wavelengths are multiplexed.
With the method for collectively amplifying optical signals of multi-wavelength signals, as only one optical amplifier using an erbium-doped fiber is required, there is a great economical advantage.
For such an optical amplifier for collectively amplifying optical signals of multiple wavelengths, however, a problem exists relative to the gain control for individual signal light outputs. The present inventors proposed a method for pumping two wavelengths in the 0.98 .mu.m and 1.48 .mu.m bands as a gain control method (e.g., "Characteristics Of EDFA For Multiple Wavelength Collective Amplification By Pumping Wavelengths In The 0.98 .mu.m and 1.48 .mu.m bands", "Proceedings of the 1995 IEICE General Conference (IEICE: the Institute of Electronics, Information and Communication Engineers), issued Mar. 10, 1995; corresponding U.S. patent application Ser. No. 508,471 was filed Jul. 28, 1995".
In each of the FIGS. 3A through 3C in the paper are shown a relationship between excitation light power and the gross output that the present inventors measured when two wavelengths in the 0.98 .mu.m and 1.48 .mu.m bands were pumped. In FIG. 1 that is an enlarged representation of the graph in FIG. 3, the horizontal axis represents a 0.98 .mu.m pumping light power, the vertical axis represents a 1.48 .mu.m pumping light power, and the gross output is plotted as the sum of two signal light outputs.
In FIG. 1, when it is assumed that the 0.98 .mu.m pumping light power is 12.5 mW and the 1.48 .mu.m pumping light power is 22.5 mW, the gross output is 11 dBm (see point P in FIG. 1). The graph in FIG. 1 was prepared by measuring and plotting such a relationship.
For the optical amplifier for collectively amplifying optical signals of multiple wavelengths, it is preferable that a gain be constant for individual wavelengths, i.e., that there be no difference in the gains for a plurality of wavelengths. In FIG. 1, .DELTA.G=0 is a characteristic line that is obtained by plotting points at which there is no gain difference between the wavelengths in the 0.98 .mu.m and 1.48 .mu.m bands.
It is therefore apparent from FIG. 1 that, when the 0.98 .mu.m pumping light power is changed from 22 mW to 50 mW on the line for gain difference .DELTA.G=0, the light output can be controlled within about a 1 dBm range (A-B) of from 12.5 dBm to 13.7 dBm.
Recently, however, a demand has arisen for a system that has an enlarged gain control range, i.e., that can control the gain within a range of 6 dBm, for example.
The previously mentioned method that the present inventors proposed does not yet fully answer this demand.